A fuel cell usually consists of a series of membrane electrode assemblies and bipolar plates stacked together in an alternating manner. The membrane electrode assembly is typically made of an ion conductive membrane sandwiched between an anode and a cathode each on the opposite side of the membrane. Bipolar plate is a plate like electric conductor having plurality of channels for fluid passage. At least two different reactive gases flow through the bipolar plate channels to reach the respective anode and cathode sections where electrochemical reactions of the gases take place to generate electricity. The electricity generated from the electrochemical reactions is collected and conducted through the bipolar plate to an external circuit. The bipolar plate, therefore, needs to have high electric conductivity or low electric contact resistance to minimize energy loss. The bipolar plate also needs to meet very stringent corrosion resistance requirement due to the harsh environment created by the reactive gases, electrochemical reactions and contaminants generated from the membrane electrolyte.
In the case of a hydrogen fuel cell, water management is another key challenge. Water is continuously generated in a hydrogen fuel cell. In addition, the fuel cell membrane needs to maintain a certain level of hydration for necessary proton conductivity. When a hydrogen fuel cell is operated at a low current density, for example, at 0.2 A/cm2, there is usually not enough gas flow to remove the water generated at the cathode section. As a result, water tends to condense in the fluid passages as droplets near the cathode and block the flow of the reactive gas. Without the supply of reactive gas, the blocked section of the fuel cell will not produce electricity. Performance of the fuel cell will deteriorate due to non-homogeneous current distribution. Such phenomenon is known as low power stability (LPS). Conventional noble metal conductive coatings, such as gold and platinum coatings, have water contact angles greater than 40 degree, a condition conducive to formation of stable water droplets. Such high water contact angles thus do not provide desirable water management.
Low cost, light weight and easy manufacturing process are also important consideration for a commercially viable and desirable bipolar plate. Metal plates are attractive bipolar plate materials due to their high electric conductivity and low cost. Metals such as stainless steel and aluminum can be easily made into very thin sheet. Fluid flow channels can be created on a metal sheet by a simple inexpensive stamping process. Most of the low cost metal sheets, however, do not have the corrosion resistance required in a fuel cell, mainly because of the corrosive fluoride ions released from the fuel cell membrane. Metal corrosion not only degrades the bipolar plate itself, but also produces soluble metal ions that contaminates the fuel cell membrane and impairs its proton conductivity. There is thus a need to provide a low cost metal bipolar plate with improved corrosion resistance and water management properties.